This application claims the priority of German patent document 199 50 008.8, filed Oct. 18, 1999 and PCT International Application No. PCT/EP00/09980, filed Oct. 11, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for controlling and for setting the switching state of a switching connection between the electrical outputs or poles of a fuel cell and an electrically isolated network which is arranged in the apparatus, and which supplies power from the fuel cell to loads that connect therewith. A further network is provided in the apparatus, whose voltage is lower than that of the network which can be connected to the fuel cell and that of electrical loads as well as that of a storage battery.
Mobile power generating systems (with fuel cells, in particular) often have two electrical networks. The network which is fed from the fuel cell is not grounded, is electrically isolated, and contains, for example, one or more drive motors for the mobile apparatus. A range of auxiliary units are required for starting and operation of the fuel cell, and their drive motors are supplied with power from a rechargeable battery while the fuel cell is being started. The rechargeable battery is arranged in a low-voltage network, and supplies the drive motors of the auxiliary units (which are arranged in a high-voltage network) with electrical power via a DC/DC converter. During operation of the fuel cell, the battery can be charged via the converter arranged between the two networks. The converter, which provides DC isolation between the two networks, feeds the loads which are connected to the low-voltage network, even during operation of the fuel cell. At least in vehicles, one pole of the low-voltage network is generally connected to the vehicle ground.
In order to avoid any danger to personnel or to parts of the mobile device resulting from current from the fuel cell, the switching connection at the output of the fuel cell either must not be closed, or must at least be opened in certain situations or when certain events occur. For example, in the case of fuel cells which are operated with hydrogen, the fuel cell system must be checked for uncontrolled escape of hydrogen before and during operation. In order to prevent a defect in the fuel cell, it is necessary to ensure that the high-voltage network is not connected to the fuel cell until it has reached operational readiness, and is quickly disconnected from it if an unacceptably large amount of current is drawn, for example in the event of a short circuit, or if the output voltage is too low. A minimum isolation resistance between the two networks must also always be maintained, for safety reasons.
The invention is therefore directed to a method and apparatus for use in a power supply system, which has a fuel cell with a switching connection for an electrically isolated network containing electrical loads and has a further network designed for a lower voltage than the fuel cell voltage, with further electrical loads. According to the invention, the switching connection between the outputs of the fuel cells and the network can either not be closed, or can be opened quickly in certain situations or certain operating states in which a conductive connection between the electrical outputs of the fuel cell and the electrically isolated network can endanger personnel, or endanger parts of the apparatus or the fuel cell itself.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which sensors arranged in or on the apparatus are used to detect and signal the existence of certain “apparatus states” in which the supply of power to the electrically isolated network must be precluded or stopped for safety reasons. The sensors are monitored by a control and evaluation unit that is supplied with power from the network. The switching connection has at least one switching or operating contact of a switching member connected to each output or pole of the fuel cell, in order to disconnect the electrically isolated network from the outputs or poles of the fuel cell that are electrically isolated from the conductive parts of the apparatus. The switching or operating contacts which are connected to the poles or outputs are part of one switching member (or of separate switching members connected to the switching and control unit). The energy for closing the switching or operating contacts during starting of the fuel cell is released to the switching member or members by the switching and control unit once the fuel cell has reached operational readiness, provided the switching and control unit does not detect any sensor signal indicating that closure of the switching or operating contacts is not permissible for safety reasons. Upon detection of a sensor signal indicating that closure of the switching or operating contacts is not permissible for safety reasons, the control and evaluation unit blocks or interrupts the power supply to the switching member or members.
The method according to the invention achieves a high degree of operational reliability for the fuel cell system. It is thus possible to prevent danger to personnel and to the environment as well as to parts of the apparatus itself. In the event of short circuits in the electrically isolated network, or in the loads fed from this network, the power supply to the switching member members fails, so that the switching or operating contacts open automatically. When a danger or defect is signalled, the fuel cell system changes to a state in which the output power from the fuel cell is interrupted, that is to say a safe operating state is reached.
The series circuit formed by a resistor and a switching or operating contact of a further switching member (which is connected to the switching and control unit and can be operated separately from the other switching members) is preferably connected in parallel with one of the switching or operating contacts which are connected to the outputs or poles of the fuel cell. Energy for closing the switching or operating contact of the further switching member is supplied at the same time as the energy for closing the switching or operating contact of the switching member after the starting of the fuel cell and after it has reached operational readiness. If the switching and control unit detects a signal which indicates that closure of the switching or operating contacts of the switching members is not permissible, power for operating the further switching member and the other switching members is blocked or interrupted. The method described above results in an initial charge between the fuel cell outputs and the electrically isolated network. The potentials on both sides of the switching member contacts are matched by closing the initial charging path, which contains the resistor, in order to largely avoid generating sparks when the contacts are closed.
In one preferred embodiment, the emergence of hydrogen from the hydrogen-producing or hydrogen-storing units is monitored by means of gas sensors. A sensor which is connected downstream from the fuel cell monitors whether the load current from the fuel cell has overshot or undershot limit values, and at least one crash sensor monitors whether the mobile apparatus has impacted with an obstruction. The isolation resistance of the electrically isolated network from the mobile apparatus earth is monitored. In addition, the output voltage of the fuel cell is monitored to determine whether limit values are overshot or undershot. Furthermore, the closure state of doors and covers is monitored by switches, and the supply voltage of the further network is monitored for limit values being overshot and undershot. Finally, the presence of conditions for the opening of the switching or operating contacts of the switching members is checked.
In particular, the isolation resistance between the electrically isolated network and the mobile apparatus ground is determined by means of a pulse measurement method, with alternate positive and negative pulses being passed to ground via a measurement resistor of predetermined magnitude. The pulses cause a current to flow via the isolation resistances to a reference point in the network; and the latter current is measured by means of the voltage drop across the measurement resistor. The measurement voltage is supplied to an A/D converter via a high-pass filter and a low-pass filter. The isolation resistance is monitored to detect any overshooting of a lower threshold (which can be predetermined) or of an excessively high threshold. The isolation measurement is carried out continuously during operation of the mobile device while, during starting, it is carried out as a quick measurement with reduced accuracy. If the isolation resistance is too low, the ungrounded network is disconnected from the fuel cell.
In another expedient embodiment, the current measurement device is monitored by feeding a test current into an additional winding of the current sensor, which contains a current transformer.
For safety reasons, in one preferred embodiment, when one or more of the gas sensors responds, the contacts of the switching connection between the outputs or poles of the fuel cell and the electrically isolated network, as well as at least one contact of an additional switching member in the further network are opened in order to interrupt the power supply at least to the switching and control unit.
In the apparatus according to the invention, the outputs or poles of the fuel cell are each connected via at least one switching or operating contact to the electrically isolated network. A control and evaluation unit of an assembly has a processor which is connected to an internal bus of the mobile apparatus, a logic circuit with logic functions in the form of hardware, an A/D converter (which is connected to the processor in a DC-isolating manner and which is connected at its input to analog sensors for the current and voltage of the fuel cell and to a measurement device for the isolation resistance between the electrically isolated network and the mobile apparatus earth), a sensor for fuel cell current which is connected to the processor and the logic circuit in a DC-isolated manner, and a power supply unit, which is fed from the further network, with DC isolation for supplying an operating voltage to the A/D convertor. Control modules are connected to the output of the logic circuit and converters that are connected to the processor and to the logic circuit, for supplying an operating voltage to the hydrogen sensors, and for matching signals which are emitted from these sensors to the levels of the logic circuit and of the processor.
The logic module, the processor, the converters and the control modules are supplied with an operating voltage from the further network. At least one output of the processor (30) is connected to a corresponding input of the logic circuit, which logic circuit is connected to sensors for producing signals relating to operating states of the mobile apparatus or its modules. The enabling of output signals from the control modules can be controlled by means of (or via) the logic circuit, which is controlled by a higher-level appliance and is connected to the respective one coil of a first switching member which has one of the two contacts and of a second switching member which has the other switching contact. For current, voltage and/or isolation resistance measured values which are detected by the processor, and for sensor measured values which are detected by the logic circuit for which the supply to the electrically isolated network must be precluded or stopped for safety reasons, the output of output signals from the control modules to the coils is blocked or interrupted.
The assembly according to the invention provides a compact controller of relatively low weight and with small dimensions for a mobile fuel cell system, by means of which numerous safety requirements can be satisfied during operation of the fuel cell system. The signals which are processed by the logic circuit relating to critical states of the fuel cell system or of the mobile device cause the switching contacts to be opened immediately.
A further switching or operating contact of a third switching member in series with a resistor is preferably arranged in parallel with the contact of the second or first switching member, and the third switching member is connected to a driver module that is supplied with an operating voltage from the further network on the assembly. The driver module is connected to the logic circuit such that, in the start phase after the fuel cell has reached operational readiness (before the emission of a control signal to the one control module, which operates the second or first switching member), the logic circuit applies a control signal to the driver module to emit an operating signal to the third switching member at the same time as the emission of a control signal for operating the first or second switching member. It also blocks the emission of control signals to operate the three switching members, in order to disconnect the outputs of the fuel cell from the electrically isolated network. The method described above results in an initial charge between the fuel cell outputs and the electrically isolated network. The initial charge causes the potentials on both sides of the contact poles to be matched to one another, thus largely avoiding the generation of sparks during switching. This protects the contacts.
When the gas sensors respond, it is also advantageous to disconnect not only the switching connection between the fuel cell and the electrically isolated network, but also the connection for the further network to the rechargeable battery. For this purpose, a further driver module, which receives the operating voltage from the further network on the assembly is connected to the output of the logic circuit, and a system relay is operated, which can be disconnected together with the switching members when at least one gas sensors responds, in order to interrupt the power supply from the further network at least for that assembly.
In one preferred embodiment, the assembly has a printed circuit board with a first section fitted with those components that are supplied with an operating voltage from the further network (such as the logic circuit, the processor, the converter and control and driver modules with associated conductor tracks) and the connections for the coils of the switching members and the connections for sensors that are supplied with the voltage of the further network, for bus conductors and for the earth connection. This first section is separated from a further section of the printed circuit board, which is fitted with those components to which the voltage of the fuel cell or the isolated electrical network is applied (such as the current sensor, a voltage divider for measuring the voltage and the associated conductor tracks as well as connections for the network fed by the fuel cell). The separation (which is formed by the two sections) of those components and conductor tracks which carry low voltages from those components and conductor tracks which carry high voltages results in a high level of safety with regard to short circuits between the two networks on the printed circuit board.
In one expedient embodiment, the negative side of the fuel cell voltage provides the reference potential for those components on the assembly to which the voltage of the fuel cell is applied; it is connected to one connection of the second section of the assembly. This reference potential allows major circuit simplification. The current sensor preferably has a current transformer with a conductor that passes through the printed circuit board and through a current transformer core arranged on the printed circuit board (without touching them). The current is measured using the compensation principle.
In particular, the current transformer is fitted with an additional winding, which is connected to a device for feeding in a defined current and is used to monitor the current measurement for functionality. Upper and lower limit values for the fuel cell current are stored in the processor. An appropriate signal is emitted to the bus, and the switching or operating contacts downstream from the poles or outputs of the fuel cell are opened if the upper limit value is exceeded, or if the lower limit is undershot.
A measurement resistor is preferably in the second section of the printed circuit board for measuring the isolation resistance of the isolated electrical network from the ground of the mobile device. Positive and negative voltage pulses applied to the measurement resistor cause a current flow, which in turn causes a voltage across the measurement resistor. The latter voltage is supplied via a high-pass filter and a low-pass filter to the A/D converter, whose output signals are supplied to the processor. The high-pass filter blocks the fuel cell DC voltage, and the low-pass filter screens out radio-frequency interference. After a specific waiting time, the processor processes the measured values for determining the isolation resistance, in order to avoid transferring any measured values during transient oscillation processes.
The processor, together with its software, is preferably monitored by a watchdog module. Thus, a processor module defect or a software crash causes the contacts at the output of the fuel cell to open, so that the initial charging path is also disconnected.
Expediently, the bus is also checked continuously for correct operation. It is advantageous for the supply voltage for the further network to be detected by means of an A/D converter, for example by means of an appropriate input to the processor, and to be monitored against predetermined limit values. If these limit values are overshot or undershot, the signal for opening the make or switching contacts is passed from the processor to the logic circuit.
In one particularly expedient embodiment, the inputs and outputs of the components arranged on the printed circuit board (such as the logic circuit, processor, control and driver stages and converter) are designed to be resistant to short circuits, resulting in a high level of operational safety.
In particular earth shorts and short circuits are detected as faults.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.